The increased use of tunable pulsed lasers in industrial, medical, and scientific applications requires safety measures to be taken to protect the eyes of users from the laser radiation.
There have been suggested safety glasses which provide a selective blocking of radiation of a specific wavelength. However, these glasses leave relatively broad regions of the spectrum open for vision and consequently are not suitable for the protection from tunable lasers over the entire visible range. Another important disadvantage of glasses of this kind is the chromatic distortion of their field of view.
To enable the eye protection in a broad range of wavelengths, it has been suggested to utilize, in safety glasses, lenses in the form of electro-optical shutters which are switched from their transparent state into their opaque state by an electrical voltage signal applied thereto.
Such safety glasses are disclosed, for example, in U.S. Pat. No. 4,560,239 and U.S. Pat. No. 5,067,795. In the glasses, an optical sensing device identifies a threatening radiation beam and activates power supply means to quickly switch the shutters into their opaque state.
U.S. Pat. No. 5,276,539 discloses safety glasses of the above kind in which the glasses transmittance varies in accordance with the intensity of radiation in their field of view. Thus, the glasses comprise two electro-optical shutters and an optical sensing device which measures the average intensity of radiation in the field of view of the shutters and activates control means to vary transmittance of the glasses by switching the shutters from their transparent to their opaque state at a relatively high frequency and for time intervals which are selectively controlled depending on the measured intensity of radiation. Thereby, the level of brightness of the view perceived by the observer is regulated. As indicated in U.S. Pat. No. 5,276,539, in order to be undetectable by the human eye, the frequency of switching the electro-optical shutters must be not less than a threshold value of 22 Hz.
In all the above disclosures, the electro-optical shutters are liquid-crystal shutters having a sufficiently short response time needed for their switching from a state of high transparency to a state of complete opaqueness and, consequently, capable of passing from one state to another at a relatively high rate. Certain liquid crystal shutters have the additional advantage that they can provide a substantially wide field of view (more than 130.degree.) and can ensure eye protection over practically the entire visible range with minimal chromatic distortion.
However, neither of the devices described above is suitable for the protection from a radiation of pulsed lasers (such as, for example, mode-locked or Q-switched lasers) in which a normal duration of pulses is extremely short, i.e. in the nano- or picosecond range, while the shortest response time which can presently be obtained with liquid crystals is about a few tens of microseconds (ferroelectric liquid crystals), not to mention the response time of conventional liquid crystal shutters which is in the order of milliseconds (supertwisted nematic (STN) or twisted nematic (TN) liquid crystals).
It is therefore the object of the present invention to provide a new method and device for the protection of human eyes, or other radiation sensitive systems, from a pulsed radiation in a relatively broad spectral region.